This invention relates to a large-aperture, wide-angle photographic lens and particularly to a lens system having a short total length.
A strong desire on the part of users of single lens reflex cameras has recently prompted attempts to construct wide-angle or ultra-wide-angle lenses which are more compact and yet have larger apertures than hitherto available Wide-angle lenses for single lens reflex cameras generally utilize a so called retrofocus arrangement. A characteristic of this type of photographic lens is that the symmetry of the lens is impaired when the lens permits a long back focal distance. This makes it difficult to correct aberrations in such lenses. The difficulty tends to increase as the lens construction incorporates a wider angle of view, a larger aperture and a smaller size.
The most difficult problem in reducing the size of a retrofocus type wide-angle lens lies in correcting the distortion in the divergent lens group at the object end of the lens. Such aberration can be effectively corrected by including a positive lens in the divergent lens group. The typical lens arrangements in such a divergent lens group include the following two: In one lens arrangement, a negative lens is disposed at the object end of the lens arrangement and a positive lens positioned behind the negative lens. In the other lens arrangement, a positive lens is disposed in one position and a negative lens arranged behind the positive lens. Because the incident height and the incident angle of light rays relative to the optical axis of the positive lens that acts to correct distortion are great, the corrective action can be made more effectively with a positive lens disposed on the object end. Hence the positive lens disposed at the object end avoids one problem, namely that the total length can not be shortened for effective correction of aberration. However, such a lens arrangement has the following disadvantages: The rays of light outside of an optical axis are excessively bent by the positive lens at the object end. This tends to increase astigmatism, comatic aberration, chromatic distortion aberration or chromatic coma aberration. Moreover, because it is necessary to increase the diameter of the first lens in view of the excessive inward bending of the rays of light, the front lens diameter tends to become greater than other lenses of the same angle of view.
An example of a retrofocus type lens with a positive lens disposed at the object end is disclosed in U.S. Pat. No. 3,877,795. Such a known lens arrangement, however, requires a plurality of negative meniscus lenses behind a positive meniscus lens. Further, the positive lens to be disposed next to the negative meniscus lenses is either a biconvex lens or a positive meniscus lens directing its convexed face toward an image of the object being photographed.